The present invention relates generally to DC--DC power converter and more specifically to invention is directed towards DC--DC converters having a half-bridge circuit topology in combination with current-mode control.
FIG. 1 shows a partial schematic diagram of a conventional DC--DC power converter 100 in a half-bridge topology. The DC--DC converter 100 includes first and second switches 110, 112, first and second capacitors 114, 116, a primary winding 120 and a secondary winding 122, and a pair of diodes 124, 126. In theory, the half bridge topology operates to maintain a voltage V.sub.in /2 at its center tap (node 128) and an output voltage V.sub.out dependent on the ratio of turns. Referring to FIG. 1, assume that the capacitance of capacitor 114 is equal to the capacitance of capacitor 116 and that switches 110 and 112 alternate being closed an equal amount of time. This results in the voltage at the center tap (node 128) being maintained at V.sub.in /2. Assuming the primary winding of the transformer has 100 turns and a secondary winding has five turns for the embodiment shown in FIG. 1, for an input voltage V.sub.in =200 volts, the center tap voltage would be 100 volts and output voltage V.sub.out at node 134 would be approximately 5 volts peak.
The half bridge topology shown in FIG. 1 is desirable as it subjects the transistors (in the example shown in FIG. 1, switches 110 and 112) in their off state to a voltage equal to the DC input voltage and not to twice that as do other topologies such as the push-pull, single-ended, and forward converter technologies. Thus, using a half-bridge topology allows the use of transistors with a lower voltage rating. An additional advantage compared to the push-pull converter topology, is that the half-bridge is immune to flux imbalance problems which plague the simplest push-pull topologies. Further, the half-bridge topology allows the use of a push-pull primary drive (which minimizes the transformer size) without requiring power-wasting snubbers. A further advantage of the half-bridge topology is it does not require a center-tapped power winding and requires only two primary switches.
Operating parameters within power converter circuits may be monitored to provide feedback that stabilizes and controls the converter circuitry. In current mode topologies, both voltage and current are typically monitored. In contrast in voltage mode topologies, the output voltage alone is monitored and controlled.
Current-mode control has several advantages compared to voltage-mode control topologies. First, current-mode control is typically is easier to stabilize than voltage mode control. In addition, current mode control gives cycle-by-cycle current limiting which protects components from certain failure modes. Further, current-mode control topologies facilitate current sharing by multiple DC--DC converters. Unfortunately, although current-mode control topologies have advantages, there have been problems combining current mode control with half-bridge circuit topologies.
The difficulty in combining current-mode control with the half-bridge circuit topolgy is that if the bridge center tap (node 118) is not held at exactly one half the input voltage, a "runaway" condition can occur. This runaway condition is typically caused when the center tap voltage is unstable, a condition typically caused by different amount of charge being removed from the capacitors 114, 116. Different amount of charge are typically removed based on uneven storage times in the switching transistors 110 and 112. If the switching transistors 110 and 112 have uneven on times, then the cycle-by-cycle current control will act to drive the center tap even farther away from one half the input voltage.
FIG. 2 shows a partial schematic diagram of a half-bridge topology modified to include a balance winding to compensate for current drift. Such a configuration is shown on pages 10-129 through 10-130 of the Unitrode Application notes. The modified half-bridge topology 200 includes first and second switches 210, 212, first and second capacitors 214, 216, a power transformer 220 which has a primary side that includes two windings a power winding 232 and a balance winding 234, and a pair of balance diodes 224, 226. In the embodiment shown in FIG. 2, the power transformer includes a power winding 232 and a balance winding 234, where the power winding 232 and the balance winding 234 both have an equal number of turns. Further, both a first terminal of the power transformer winding and a first terminal of the balance winding are connected to the center tap node 235. In addition, the second terminal of the balance winding is connected through the balance diodes 224, 226 to each supply rail. In this configuration, the center tap node is forced to 1/2V.sub.in by nature of the identical number of turns on the power and balance windings. Should the center tap voltage begin to drift away from 1/2V.sub.in, current will flow through the balance winding to compensate by allowing the balance diodes 224 or 226 to conduct to charge either capacitor 214, 216 so as to bring the center tap voltage towards 1/2V.sub.in.
Although in theory, the half-bridge configuration shown in FIG. 2 compensates for current drift, in practice Applicant has found that it will not prevent a runaway condition in some applications useful to Applicant. Applicant believes the resulting nonoperational state is at least in part due to having a low input voltage (typically, a voltage less than 50 volts). The lower the input voltage, the lower the sensitivity of the balance circuit because of the fixed forward voltage drop of the balance diodes.
A DC--DC converter which combines half-bridge topology with a current-mode control that provides a stable center tap voltage under a variety of operating conditions is needed.